The writing process for a holographic recording medium is implemented by irradiating a signal beam with a superimposed image information and a reference beam onto a recording layer so as to record the image information as an interference pattern. The reading process for the holographic recording medium is implemented by irradiating the reference beam onto the recording layer where the image information is written so as to read the image information out.
In the holographic data storage, the image information can be recorded and read out as one page and thus at one time per page so that a plurality of pages relating to respective image informations can be multiply recorded in the same area of the recording layer of a holographic recording medium. Therefore, the holographic recording technique is a promising candidate for a next-generation optical disc technology with a high data transfer rate and a large capacity which can be substituted for a conventional bit-by-bit data storage technology which is employed in conventional CDs, DVDs and Blu-ray Discs.
The holographic recording medium can be largely classified in a transmission-type holographic medium and a reflection-type holographic medium. In the transmission-type holographic medium, an optical beam is passed through the holographic medium with no reflective layer so as to conduct the recording and reproducing process. In the reflection-type holographic medium, an optical beam is reflected at a reflective layer provided for the holographic medium so as to conduct the recording and reproducing process.
Moreover, the transmission-type holographic medium can be classified into two types of holographic media. In one transmission-type holographic medium, a signal beam and a reference beam are irradiated onto a holographic recording medium from the same side thereof for recording and the reference beam is irradiated onto the recorded area of the holographic recording medium so that the thus holographically diffracted beam can be passed through the holographic medium for reproducing (belonging to the transmission-type hologram on holographic classification). In the other transmission-type holographic medium, a signal beam and a reference beam are irradiated onto a holographic recording medium from the opposite side thereto for recording and the reference beam is irradiated on the recorded area of the holographic recording medium so that the thus holographically diffracted beam can be reflected from the holographic medium for reproducing (belonging to the reflection-type hologram on holographic classification).
In such a transmission-type holographic medium, a reference beam can be irradiated onto the recorded area of the holographic medium for reproducing from the side opposite to the side where the signal beam and the reference beam are irradiated onto the holographic medium for recording. In this case, the thus obtained reproduced signal can be rendered an optical signal of phase conjugation. This reproducing process is called as “phase-conjugate reproduction” whereby the optical distortion, typified by the lens aberration, in the optical components of the optical system can be almost cancelled at the reproducing process. Therefore, the reproducing process becomes efficient particularly as a reproducing means for reproducing an image information with a higher S/N ratio under the condition of low distortion. In the use of the transmission-type holographic medium, since the corresponding recording/reproducing optical systems are required to be positioned at both sides of the holographic medium in both of the transmission-type hologram and the reflection-type hologram, the total recording/reproducing optical system becomes a largely scaled and complicated one.
In such a reflection-type holographic medium, on the other hand, the corresponding recording/reproducing optical system is required to be positioned at either side of the holographic medium, the total recording/reproducing optical system becomes a small scaled and simplified one. In this point of view, it is advantageous to employ the reflection-type holographic medium in the case where the holographic recording technology is applied for optical discs. In the reflection-type holographic medium, however, some optical beams with noise superimposed thereto may be generated due to the reflection beams in both of the recording process and the reproducing process so that the treatment of the optical beams with noise become one of the problems to be solved. The holographic medium and the recording/reproducing method to solve such a problem are proposed in Patent document No. 1 and Non-patent document No. 1.
Patent document No. 1 teaches a holographic reproducing technique where a reflective layer is formed on a main surface of a holographic recording medium to which a recording signal beam and a recording reference beam are to be irradiated, and a reproducing reference beam is irradiated on the other main surface of the holographic recording medium with no reflective layer.
Moreover, Non-patent document No. 1 teaches a holographic recording medium where a phase-change layer and a holographic recording layer are subsequently stacked. In the implementation of the holographic recording process, the phase-change layer is rendered amorphous so as to lower the reflectivity and in the implementation of the holographic reproducing process, the phase-change layer is rendered crystalline so as to increase the reflectivity. In this case, the formation of unnecessary diffraction grating can be reduced, resulting in the provision of the holographic recording medium with a high S/N ratio and a large multiplexing number.